Method for Producing Polishing Agent, Polishing Agent Produced Thereby and Method for Producing Silicon Wafer

ABSTRACT

The present invention is a method for producing a polishing agent in which silica particles are dispersed in an aqueous solution, comprising at least a step of removing metal compound ions from a prepared silica sol by ion exchange (B); a step of purifying further the ion-exchanged silica sol (D); a step of adding alkali metal hydroxide to the purified silica sol (F); and a step of adding an acid to the silica sol to which the alkali metal hydroxide is added (G). There is provided a method for producing a polishing agent which can extremely effectively suppress metal contamination when a silicon wafer or the like is polished can be produced.

TECHNICAL FIELD

The present invention relates mainly to a method for producing apolishing agent, particularly a method for producing a polishing agentsuitable for polishing a silicon wafer.

BACKGROUND ART

Semiconductor wafers used for manufacturing semiconductor devices areproduced through various steps. For example, when a silicon wafer isproduced, a silicon single crystal ingot grown by Czochralski method orthe like is sliced and processed into a disk-shaped wafer (slicingstep). Subsequently, a peripheral edge portion of the wafer is chamferedin order to prevent breaking or cracking (chamfering step), then lappingis carried out in order to flatten a surface of the wafer (flatteningstep). There are some cases where surface grinding is carried outinstead of lapping. Then, etching is carried out in order to removemechanical damages and the like left on the surface of the wafer(etching step).

Moreover, surface polishing is carried out in order that the surface ofthe wafer is flattened further and made a mirror surface at the sametime (polishing step). Thereafter, contaminants such as a polishingagent and a foreign body attaching to the surface of the wafer duringthe polishing are removed using a cleaning solution containing hydrogenperoxide and the like (SC-1, SC-2 and so on) (cleaning step). Lastly asilicon wafer produced after these steps is subjected to a qualityexamination and the like, then put in a dedicated container andpackaged, then shipped to a device maker (device manufacturing process).

There are some cases where steps of heat treatment and the like areadded to the above steps, the order of the steps is changed, or the samestep is carried out with it divided into plural steps.

In the polishing step, a wafer is polished using, for example, apolishing apparatus 1 shown in FIG. 2. In this apparatus 1, a wafer W isheld with polishing head (holding plate) 2, then contacted with apolishing pad 4 attached to a polishing turn table 3 with a givenpressure and polished. On this occasion, the polishing pad 4 is providedwith a polishing agent 6 through a nozzle 5 while the polishing head 2and a polishing turn table 3 are respectively rotated on rotation axes 8and 7. In polishing a silicon wafer, a polishing agent in a state ofsuspension (also called a slurry) is generally used, wherein an alkalinesolution is added to a colloidal silica (a silica sol) in whichfine-particle silica (SiO₂) is suspended in pure water. On the groundthat such a polishing agent intervenes between a polishing pad and asilicon wafer, mechanochemical effects are brought about on thepolishing agent and the wafer and polishing is promoted. Additionally,there are some cases where the polishing step is divided into the firstpolishing step in which a relatively rough polishing is carried out anda final polishing step in which more precise polishing is carried out,or according to circumstances, the polishing step is divided into threeor more steps such as the first step, the second step and a final step.

Recently, as a device becomes finer, higher quality for a semiconductorwafer is demanded. Preventing metal contamination as possible along withfinishing a wafer surface in high flatness is required. For example, aCu concentration of a polished wafer is required to be a level of 1E+10atoms/cm² or less. Therefore, in the polishing step, it is necessary touse a polishing agent containing as few metal impurities as possible.

As a method for producing a silica sol used for a polishing agent, thereis a method in which metal impurities are removed by ion exchange andthe like to improve the purity (see Japanese Patent ApplicationLaid-opens (kokai) Nos. 5-97422 and 6-16414). For example, there isdisclosed a method for producing an aqueous silica sol containingsubstantially no metal impurities (metal oxides) in which an aqueoussolution of alkali metal silicate is contacted with a cation exchangeresin; subsequently a strong acid is added, ion exchange is carried out,alkali metal hydroxide is added and so on to provide an aqueous silicasol; then ion exchange is carried out and ammonia is added to produce anaqueous silica sol containing substantially no metal impurities (metaloxides) (see Japanese Patent Application Laid-open (kokai) No. 5-97422).

Moreover, as a polishing agent used for polishing a surface of asemiconductor wafer such as a silicon wafer, there is disclosed apolishing agent containing silica particles coated with aluminum oralumina. It is said that a colloidal silica containing silica particlescoated with aluminum has high stability in a strong alkaline solution(see Japanese Patent Application Laid-open (kokai) No. 11-302635).

Recently, a silica sol coated with alumina is widely used. To produce apolishing agent containing such an alumina-coated silica sol, generally,a sodium aluminate sol is mixed with a silica sol purified by ionexchange and the like, then heat sterilization treatment is carried out.Thereby silica particles are alumina-coated to provide a polishing agenthaving an outstanding dispersibility.

However, there has been a problem that in the case where a silicon waferis polished using the polishing agent produced through the steps such asion exchange and alumina-coating as described above, a concentration ofmetal impurities such as Cu in the bulk of a wafer becomes high and therecent demand for quality of device manufacturing cannot be satisfied.

DISCLOSURE OF THE INVENTION

The present invention was conceived in view of the above problem. Themain object of the present invention is to provide a method forproducing a polishing agent which can extremely effectively suppressmetal contamination when a silicon wafer or the like is polished.

To achieve the above-mentioned object, according to the presentinvention, there is provided a method for producing a polishing agent inwhich silica particles are dispersed in an aqueous solution, comprisingat least a step of removing metal compound ions from a silica sol by ionexchange; a step of purifying further the ion-exchanged silica sol; astep of adding alkali metal hydroxide to the purified silica sol; and astep of adding an acid to the silica sol to which the alkali metalhydroxide is added.

As described above, when a polishing agent is produced after an aqueoussolution in which silica particles are dispersed is ion exchanged,purified further, added alkali metal hydroxide, and added an acid inthat order, metal ions such as Cu attaching to the peripheries of silicaparticles can be effectively removed, so that a polishing agent havingextremely high purity can be produced.

It is preferable that a strong acid is used as the acid.

In particular, when a strong acid such as sulfuric acid, nitric acid orhydrochloric acid is used, there can be produced a polishing agent towhich an effect of suppressing metal contamination such as Cu is added.

It is preferable that after the alkali metal hydroxide is added,quaternary ammonium salt is added either together with the acid, orbefore or after the acid is added. Particularly, it is preferable thattetramethylammonium hydroxide is used as the quaternary ammonium salt.

As described above, when quaternary ammonium salt, particularlytetramethylammonium hydroxide (TMAH) is added, agglomeration of silicaparticles can be sufficiently prevented and a stable polishing agentwhich has an outstanding dispersibility as well as high purity.

Furthermore, after the alkali metal hydroxide is added, an organicchelating agent can be added either together with the acid, or before orafter the acid is added. Particularly, it is preferable that at leastone selected from the group consisting of polyamino-polycarboxylic acidderivatives and salts of polyamino-polycarboxylic acid derivatives isused as the organic chelating agent.

As described above, when the organic chelating agent, particularlypolyamino-polycarboxylic acid derivatives or salt ofpolyamino-polycarboxylic acid derivatives is added, metal ions such asCu in a polishing agent can be captured, and there can be provided apolishing agent to which even higher effect of suppressing metalcontamination of a silicon wafer or the like is added.

Ion exchange can be carried out as the step of purifying.

Namely, when a silica sol is subjected to ion exchange again after thefirst ion exchange, metal ions and so on left in the silica sol can beeven more surely removed, and the silica sol can be purified further.

It is preferable that an alkaline silica sol having a pH in the range of8 to 13 is used as the prepared silica sol, moreover, it is preferablethat a silica sol containing spherical-silica particles having anaverage particle size in the range of 11 to 13 nm is used as theprepared silica sol.

When such a silica sol as described above is used, a polishing agentsuitable particularly for polishing a silicon wafer is easy to beproduced.

Moreover, it is preferable that after the ion exchange is carried out,the silica sol has a pH in the range of 2 to 4, and it is preferablethat the silica sol after the acid is added has a pH in the range of 10to 12.

When a pH is adjusted as described above in each step, a silica solhaving more preferred mechanochemical effects on a silicon wafer can besurely produced.

Furthermore, according to the present invention, there is provided apolishing agent produced by the method described above.

A polishing agent produced by the method in the present invention hasextremely low concentration of metal impurities such as Cu attaching tosurfaces of silica particles, so that it is suitable particularly forpolishing a semiconductor wafer such as a silicon wafer.

Moreover, according to the present invention, there is provided a methodfor producing a silicon wafer comprising at least a polishing step,wherein as the polishing step, the wafer is polished by contacting andrubbing with a polishing pad with providing the polishing agent producedby the above-described method to the polishing pad.

When a silicon wafer is polished using a polishing agent produced by themethod in the present invention, there are extremely few metalimpurities such as Cu attaching to surfaces of silica particles,therefore, there can be produced a mirror wafer which satisfies therecent demand for quality of device manufacturing.

According to the present invention, metal ions such as Cu attaching tothe peripheries of silica particles are effectively removed, so thatthere can be produced a polishing agent having extremely high purity.Therefore, when a silicon wafer or the like is polished using thepolishing agent, metal contamination of the wafer due to the polishingagent can be effectively suppressed and there can be provided a mirrorwafer of high quality.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a flow chart showing an example of a method for producing apolishing agent according to the present invention.

FIG. 2 is a schematic view showing an example of a single-side polishingapparatus.

FIG. 3 is a schematic view showing an example of a double-side polishingapparatus.

FIG. 4 is a graph showing a Cu concentration of a silicon wafer afterhaving been polished in Example 1 and Comparative Example 1.

FIG. 5 is a graph showing a Cu concentration of a silicon wafer afterhaving been polished in Examples 1 and 2.

FIG. 6 is a graph showing a polishing rate of a polishing agent ofExamples 3, 4 and Comparative Example 1.

FIG. 7 is a graph showing the number of defects on a surface of asilicon wafer after having been polished with a polishing agent ofExamples 3, 4 and Comparative Example 1.

FIG. 8 is a graph showing a Cu concentration of a silicon wafer afterhaving been polished with a polishing agent of Examples 3, 4 andComparative Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a method for producing a polishing agent and a method forpolishing a silicon wafer using the polishing agent produced therebywill be explained specifically.

The present inventors have studied a polishing agent for a siliconwafer.

When a silicon wafer is polished using a polishing agent containing acolloidal silica, there are some cases where the wafer is metalcontaminated by metal impurities contained in the polishing agent. It isconsidered to be a cause for this kind of metal contamination that metalimpurities such as Cu present on surfaces of silica particles arediffused in the wafer due to heat generation during polishing and otherfactors, and taken into the bulk.

Then, in order to remove metal impurities such as Cu contained in acolloidal silica, it is considered to change a producing method and soon of silica particles themselves to improve the purity. However, whenthe purity is attempted to be improved in this way, shapes of silicaparticles change into distorted shapes, and when a silicon wafer ispolished using a polishing agent containing the distorted silicaparticles, an extraordinary high incidence of LPDs (Light Point Defects,a general term for defects measured by light scattering method) isbrought about in the wafer after being polished.

The present inventors have studied further, and as a result, they havefound that in producing a polishing agent, metal impurities such as Cuattaching to surfaces of silica particles can be effectively removed toproduce a polishing agent of high quality having no distorted silicaparticles, when a silica sol is purified by ion exchange and the like tosome extent, subsequently an acid is added at the final stage withoutconventional addition of sodium aluminate and a subsequent heatsterilization treatment, then, they have completed the presentinvention.

FIG. 1 is a flow chart showing an example of a method for producing apolishing agent according to the present invention. When a polishingagent is produced according to the present invention, firstly a silicasol is prepared (FIG. 1(A)). In order to make the silica sol a polishingagent that provides enhanced mechanochemical effects, it is preferablethat the silica sol is alkaline with a pH in the range of 8 to 13, andan average particle size of silica particles contained in the silica solis in the range of 11 to 13 nm. Such a silica sol can be obtained in amarket or manufactured, for example, by the following method known tothe public.

An aqueous solution of sodium silicate containing silica (SiO₂) by 1 to6% by weight is subjected to a sodium removal treatment. For example, byletting the aqueous solution of sodium silicate through a column packedwith a cation exchange resin in hydrogen form, metal ions such as sodiumions can be removed. Thereby, an aqueous solution of activated silicafrom which alkali metal ions are particularly removed can be obtained.It is preferable that a particle size of the silica particles at thisstage is about 1 to 5 nm, particularly about 2 nm.

After the sodium removal treatment is carried out as described above,alkali metal hydroxide, preferably an NaOH aqueous solution is mixedwith the aqueous solution of the activated silica. Here, a particlesize, a shape and a particle size distribution of the silica can beadjusted by temperature, time, a ratio of SiO₂/Na₂O, and so on. Forexample, by stirring the aqueous solution of the activated silica for 1minute to 12 hours at the temperature of 60 to 150° C. after adding theNaOH aqueous solution, an aqueous sol in which the silica particles aregrown to have an average particle size of about 5 to 30 nm andstabilized can be obtained. Subsequently, by letting this aqueous solthrough a porous film to remove water, a silica sol in which SiO₂concentration is heightened can be obtained.

Metal compound ions in the silica sol prepared as described above areremoved by ion exchange (FIG. 1(B)). For example, by contacting thesilica sol with a cation exchange resin in hydrogen form, metal impurityions, particularly heavy metal ions such as Cu, Fe and Ni on thesurfaces of the silica particles or in the solution can be effectivelyremoved. Additionally, since not only the metal impurity ions but alsoalkali metal ions such as Na are removed by this ion exchange, a pH ofthe silica sol changes toward the acid side. It is preferable that thision exchanged silica sol is an acid silica sol (I) having a pH in therange of 2 to 4 (FIG. 1(C)).

Subsequently, the ion exchanged silica sol is purified further (FIG.1(D)). As a means of such a purifying, for example, ion exchange can becarried out again. Namely, by carrying out ion exchange again, itbecomes possible that metal impurity ions, particularly heavy metal ionssuch as Cu left on the surfaces of the silica particles or in thesolution in minuscule quantities are removed all the more and the silicasol is purified further.

It is preferable that the silica sol after the purifying step is also anacid silica sol (II) having a pH in the range of 2 to 4 (FIG. 1(E)).

Subsequently, alkali metal hydroxide is added to the silica sol purifiedas described above (FIG. 1(F)). For example, an NaOH aqueous solutionhaving high purity is added to raise a pH of the silica sol close to therange of 8 to 14. By making the silica sol strong alkaline like this, itbecomes possible to improve its etching effect. KOH and the like can beused as the alkali metal hydroxide.

Then, as the last step, an acid is added to the silica sol to which thealkali metal hydroxide is added as described above (FIG. 1(G)). It ispreferable that a strong acid is used as the acid added here. Forexample, inorganic acids such as hydrochloric acid, sulfuric acid andnitric acid are preferably used. Although actions and advantages causedby addition of such a strong acid have not been made clear, it isconsidered that when a strong acid is added, metal impurities in a wafercan be reduced through such a mechanism as follows.

As the silica sol before the acid is added is alkaline, metal impuritiesand so on are present in the form of metal hydroxides (for example,Cu(OH)₂, Ni(OH)₂). Since these metal hydroxides are positively charged,they are firmly adsorbed on surfaces of silica particles (a colloidalsilica) that are negatively charged. Therefore, it is considered thatwhen a silicon wafer is polished using a polishing agent containing suchsilica particles, metal impurities adsorbed on the surfaces of thesilica particles may become a cause for metal contamination. And, it isconsidered that if these metal hydroxides are kept away from the surfaceof the colloidal silica, the metal contamination can be reduced.

Then, when a strong acid, for example, sulfuric acid is added, assulfuric acid is more acidic than silica, it strips the metal hydroxideswhich are adsorbed on the surfaces of the silica particles, and isfirmly adsorbed on the surfaces of the silica particles. As a result,addition of a strong acid provides an effect of keeping metal hydroxidesaway from the surfaces of the silica particles, so that it is consideredthat the metal contamination of the wafer can be suppressed.

Furthermore, not only the strong acid as described above but a weak acidsuch as phosphoric acid can be used. Moreover, using carboxylic acidssuch as acetic acid, formic acid, oxalic acid, tartaric acid and benzoicacid, or organic acids such as sulfamic acid also can provide the effectof reducing Cu and the like. Alternatively, a mixed acid of the acids asdescribed above also can be used.

Furthermore, in this step, it is preferable that quaternary ammoniumsalt, particularly, tetramethylammonium hydroxide (TMAH) is addedtogether with the acid (FIG. 1(G)). Moreover, if the TMAH is directlyadded to an acid silica sol, it is feared that the silica sol mixed withthe TMAH becomes thickened due to a problem concerning stability ofcompatibility. Accordingly, for example, given quantity of NaOH havinghigh purity is added in advance until a pH of the silica sol becomesabout 8.5, then, sulfuric acid and the TMAH are added together. Byadding the TMAH like this, agglomeration of the silica particles can beprevented and the polishing agent can be stabilized. Furthermore,tetraethylammonium hydroxide (TEAH), methyltriethylammonium hydroxide,tetrapropylammonium hydroxide, tetrabutylammonium hydroxide,methyltributylammonium hydroxide, cetyltrimethylammonium hydroxide,choline, trimethylbenzylammonium hydroxide and so on are mentioned asthe quaternary ammonium salt that is added.

Moreover, the quaternary ammonium salt as described above can be addedeither before or after the acid is added after the alkali metalhydroxide is added.

Further, an organic chelating agent such as amine salt, ethylenediaminecan be added as occasion demands (FIG. 1(G)). Polyamino-polycarboxylicacid derivatives and salts of polyamino-polycarboxylic acid derivativescan be preferably used as the organic chelating agent. Specifically, atleast one out of nitrilotriacetic acid (NTA), hydroxyethyliminodiaceticacid (HIDA), ethylenediaminetetraacetic acid (EDTA),hydroxyethylethylenediaminetriacetic acid (EDTA-OH),1,3-diaminopropanetetraacetic acid (DPTA), diethylenetriaminepentaaceticacid (DTPA), triethylenetetraminehexaacetic acid (TTHA),2-hydroxy-1,3-diaminopropanetetraacetic acid (DPTA-OH),nitrilotrimethylenephosphonic acid (NTMP), nitrilotriethylenephosphonicacid (NTEP), ethylenediaminetetramethylenephosphonic acid (EDTMP),ethylenediaminetetraethylenephosphonic acid (EDTEP),diethylenetriaminepentamethylenephosphonic acid (DTPMP),diethylenetriaminepentaethylenephosphonic acid (DTPEP),triethylenetetraminehexamethylenephosphonic acid (TTHMP) andtriethylenetetraminehexaethylenephosphonic acid (TTHEP) can be added.

Also the organic chelating agent as described above can be addedtogether with the acid such as sulfuric acid, or can be added eitherbefore or after the acid is added after the alkali metal hydroxide isadded. When such an organic chelating agent is added, the chelatingagent captures metal ions such as Cu, so that the metal contamination ofa wafer can be suppressed.

Moreover, either of the quaternary ammonium salt and the organicchelating agent as described above, or both of the two can be added.

However, in all cases, it is preferable that in the end, the silica solafter the acid is added has a pH in the range of 10 to 12 (FIG. 1(H)).When the silica sol is alkaline with a pH in this range, it comes tohave an adequate etching effect on the silicon wafer and show preferredmechanochemical effects.

Through the steps as described above, it is possible to remove metalimpurities adsorbed on surfaces of silica particles, by adding an acid,particularly a strong acid such as sulfuric acid in the final stagewithout adding aluminate and without carrying out a heat sterilizationtreatment. Furthermore, since all that required is carrying out apurifying treatment by ion exchange and the like plural times withmaking almost no change to steps of producing silica particlesthemselves, it becomes possible to prevent the silica particles frombeing distorted, so that the silica particles can have the same shape asconventional ones. Therefore, a polishing agent of high quality in whichthe surfaces of the silica particles have extremely high purity and aparticle shape is spherical can be produced.

Next, there will be explained a case where a surface of a silicon waferis polished using the polishing agent produced by the method asdescribed above.

A polishing apparatus is not particularly limited. For example, asingle-side polishing apparatus 1 as shown in FIG. 2 can be used. When asilicon wafer W is polished, the wafer W is held with a polishing head2, then, a polishing turn table 3 and the polishing head 2 arerespectively rotated in given directions. A polishing pad 4 is providedwith a polishing agent 6 through a nozzle in a given flow rate, and thewafer W is held against the polishing pad 4 under a given load. Thereby,the wafer W is contacted and rubbed with the polishing pad 4 with thepolishing agent intervening between the wafer W and the polishing pad 4,and silica particles contained in the polishing agent contact with asurface of the wafer W along with etching due to being alkaline, thenthe polishing progresses. Here, in the present invention, the polishingpad is provided with the polishing agent produced by the method in thepresent invention.

As described above, when the wafer is polished with the polishing agentin the present invention provided in a polishing step, metalcontamination of the wafer W due to the polishing agent can be extremelyeffectively prevented, for there are few metal impurities attaching tothe surfaces of the silica particles. Furthermore, in the polishingagent produced by the present invention, since the silica particles arenot distorted and almost spherical, an extraordinary high incidence ofLPDs of the wafer will not be brought about. Accordingly, a mirrorsilicon wafer having high flatness in which metal contamination such asCu is extremely suppressed can be produced.

Moreover, the polishing can be carried out using a double-side polishingapparatus. For example, in a double-side polishing apparatus 11 as shownin FIG. 3, a wafer W held in a holding hole of carrier 12 is sandwichedbetween polishing pads 14 a and 14 b that are respectively attached on aupper turn table 13 a and a lower turn table 13 b, then the both sidesof the wafer w are polished at the same time. In this case, by providingthe polishing agent in the present invention through a polishingagent-providing hole 15 formed in the upper turn table 13 a, a siliconwafer of high quality having high flatness in which metal contaminationis extremely suppressed can be produced.

Hereinafter, Examples of the present invention and Comparative exampleswill be explained.

EXAMPLE 1

Ion exchange was carried out on an alkaline silica sol having a pH of 10(average particle size: about 11 nm) using a column filled with a cationexchange resin in hydrogen form, and ion exchange was carried out againso that the silica sol could be purified further. By carrying out theion exchange twice, there was obtained an acid silica sol having a pH of2.5. Next, an NaOH aqueous solution having high purity was mixed toincrease the pH to 12, then phosphoric acid was added to provide apolishing agent (pH 11.5).

A silicon wafer was polished using the polishing agent thus produced. Asthe silicon wafer, a silicon wafer (CW) of P-type having a diameter of200 mm, crystal axis of <100> and resistivity of 0.01˜0.1 Ωcm, that hadbeen etched after being lapped was used. The polishing was carried outwith a single-side polishing apparatus as shown in FIG. 2 with itdivided into three steps, the first step, the second step and a finalstep. Stock removal was about 10 μm in total.

After the polished wafer was subjected to SC1 cleaning and SC2 cleaning,a Cu concentration in the bulk of the wafer was measured. The Cuconcentration was measured in a way in which concentrated sulfuric acidwas dropped on the surface of the silicon wafer so that solid-solutemetal impurities in the wafer were extracted in the concentratedsulfuric acid, then Cu in the concentrated sulfuric acid was measuredwith a frameless Atomic Absorption Spectrometer (AAS) (see JapanesePatent Laid-Open (kokai) No. 2001-208743). The measurement of the Cuconcentration was carried out on two wafers.

COMPARATIVE EXAMPLE 1

Ion exchange was carried out once on an alumina-coated silica sol toprovide a polishing agent. A silicon wafer was polished using thispolishing agent. Polishing and cleaning were carried out under the samecondition as Example 1 other than the polishing agent, then a Cuconcentration in the bulk of the wafer was measured in the same manneras Example 1.

The measurement results of the Cu concentration in Example 1 andComparative example 1 are shown in Table 1 and FIG. 4.

TABLE 1 Alu- Ion Cu concentration mina ex- in the bulk coat change Addedacid (atoms/cm²) Example 1 Absent Twice phosphoric 1.83E+10 1.65E+10acid Compar- Pres- Once None 6.77E+10 6.40E+10 ative ent Example 1

As is obvious from Table 1 and FIG. 4, it was found that the Cuconcentration in the bulk is 2E+10 or less in Example 1, which isone-third of that in Comparative example 1.

EXAMPLE 2

Ion exchange was carried out on an alkaline silica sol (average particlesize: about 11 nm) in the same manner as Example 1, and ion exchange wascarried out again so that the silica sol could be purified further.Next, an NaOH aqueous solution having high purity was mixed, thensulfuric acid was added to produce a polishing agent (pH 11).

A silicon wafer was polished using this polishing agent. Polishing andcleaning were carried out under the same condition as Example 1 otherthan the polishing agent, then a Cu concentration in the bulk of thewafer was measured.

The Cu concentration in the bulk measured in Example 2 is shown in Table2 and FIG. 5 together with the measurement result of Example 1.

TABLE 2 Alu- Ion Cu concentration mina ex- in the bulk coat change Addedacid (atoms/cm²) Example 1 Absent Twice phosphoric 1.83E+10 1.65E+10acid Example 2 Absent Twice sulfuric 1.00E+10 9.50E+09 acid

As is obvious from Table 2 and FIG. 5, in Example 2 in which sulfuricacid was added, the Cu concentration in the bulk was 1E+10 or less, fromwhich it was found that the polishing agent of Example 2 had even higherdegree of effectiveness in suppressing Cu contamination than that ofExample 1 in which phosphoric acid was added.

EXAMPLES 3 AND 4

An NaOH aqueous solution having high purity was mixed after the samesteps as Example 2 such as ion exchange, then TMAH was added togetherwith sulfuric acid to produce a polishing agent (pH 11) (Example 3).

Moreover, TMAH was added together with sulfuric acid through the samesteps as Example 3, then 2-hydroxy-1,3-diaminopropanetetraacetic acidwas added as an organic chelating agent to produce a polishing agent(Example 4).

As for each of Examples 3, 4, the number of ion exchanges, the presenceor absence of an added acid and a chelating agent, and so on are shownin Table 3, together with those of the polishing agent of Comparativeexample 1.

TABLE 3 Alumina Ion Added coat exchange acid Chelating agent Compara-Present Once None None tive Example 1 Example 3 Absent Twice sulfuricNone acid Example 4 Absent Twice sulfuric 2-hydroxy-1,3- aciddiaminopropanetetraacetic acid

A silicon wafer was polished using the polishing agents of Examples 3and 4 respectively. Polishing and cleaning were carried out under thesame condition as Example 1 other than the polishing agents, then apolishing rate, the number of defects on the surface of the wafer and aCu concentration in the bulk were measured.

As for the polishing rate, stock removal was determined with anelectrical capacitance instrument for measuring thickness, and thepolishing rate was calculated with regarding the average value by thehour in Comparative example 1 as 1.0.

FIG. 6 shows the polishing rate (the average value of that of the twowafers) when each polishing agent was used. It was found that in thecase where the polishing agent of Example 3 or 4 was used, the polishingrate was improved by 20% or more, compared with the case where aconventional polishing agent (Comparative example 1) was used. This isconsidered to be an effect produced by addition of the TMAH.

The numbers of defects on surfaces of the polished wafers were measuredwith a microscope of laser confocal optics (manufactured by LasertecCorporation: MAGICS).

FIG. 7 shows the numbers of defects on the surfaces (the average valueof that of the two wafers) when the average value in Comparative example1 was regarded as 1.0. In the case where the polishing agent of Example3 or 4 was used, a density of defects on the surface of the polishedwafer was half or less than that in the case where the conventionalpolishing agent (Comparative example 1) was used. Particularly, in thecase where the polishing agent of Example 4 in which the chelating agentwas added was used, the density was about 30% of that in Comparativeexample 1.

The Cu concentration in the bulks of the polished wafers was shown inTable 4 and FIG. 8.

TABLE 4 Cu concentration in the bulk (atoms/cm²) Comparative example 16.77 · E+10 6.40 · E+10 Example 3 7.50 · E+09 6.80 · E+09 Example 4 1.40· E+09 1.40 · E+09

In the case where the polishing agent of Example 3 or 4 was used, the Cuconcentration in the bulk of the polished wafer was a digit smaller thanthat in the case where the polishing agent of Comparative example 1 wasused, and particularly, in the case where the polishing agent of Example4 in which the chelating agent was added was used, the Cu concentrationbecame still smaller than that in the case where the polishing agent ofExample 3 in which the chelating agent was not added was used, fromwhich it was found that the polishing agent of Example 4 has anextremely high degree of effectiveness in suppressing Cu contamination.

The present invention is not limited to the above-described embodiments.The above-described embodiments are mere examples, and those having thesubstantially same constitution as that described in the appended claimsand providing the similar action and advantages are included in thescope of the present invention.

For example, in the above-described embodiments, the case of polishing asurface of a silicon wafer was explained. However, the polishing agentproduced in the present invention can be used for polishing a chamferedportion.

Moreover, a polishing object is not limited to a silicon wafer, and thepolishing agent produced in the present invention can be preferably usedfor polishing various kinds of precision substrates such as othersemiconductor wafers or a quartz substrate that are required to preventmetal contamination as well as to have high flatness.

Furthermore, a silicon wafer of low resistance was used in Examples.However, the polishing agent produced in the present invention can beused regardless of resistance of a wafer and provide the same effectsnot only on reducing metal contamination such as Cu of a polished waferbut also on improving a polishing rate and reducing the number ofdefects on a surface of a wafer.

1-13. (canceled)
 14. A method for producing a polishing agent in whichsilica particles are dispersed in an aqueous solution, comprising atleast a step of removing metal ions from a prepared silica sol by ionexchange; a step of purifying further the ion-exchanged silica sol; astep of adding alkali metal hydroxide to the purified silica sol; and astep of adding an acid to the silica sol to which the alkali metalhydroxide is added.
 15. The method for producing a polishing agentaccording to claim 14, wherein a strong acid is used as the acid. 16.The method for producing a polishing agent according to claim 14,wherein after the alkali metal hydroxide is added, quaternary ammoniumsalt is added either together with the acid, or before or after the acidis added.
 17. The method for producing a polishing agent according toclaim 15, wherein after the alkali metal hydroxide is added, quaternaryammonium salt is added either together with the acid, or before or afterthe acid is added.
 18. The method for producing a polishing agentaccording to claim 16, wherein tetramethylammonium hydroxide is used asthe quaternary ammonium salt.
 19. The method for producing a polishingagent according to claim 17, wherein tetramethylammonium hydroxide isused as the quaternary ammonium salt.
 20. The method for producing apolishing agent according to claim 14, wherein after the alkali metalhydroxide is added, an organic chelating agent is added either togetherwith the acid, or before or after the acid is added.
 21. The method forproducing a polishing agent according to claim 19, wherein after thealkali metal hydroxide is added, an organic chelating agent is addedeither together with the acid, or before or after the acid is added. 22.The method for producing a polishing agent according to claim 20,wherein at least one selected from the group consisting ofpolyamino-polycarboxylic acid derivatives and salts ofpolyamino-polycarboxylic acid derivatives is used as the organicchelating agent.
 23. The method for producing a polishing agentaccording to claim 21, wherein at least one selected from the groupconsisting of polyamino-polycarboxylic acid derivatives and salts ofpolyamino-polycarboxylic acid derivatives is used as the organicchelating agent.
 24. The method for producing a polishing agentaccording to claim 14, wherein ion exchange is carried out as the stepof purifying.
 25. The method for producing a polishing agent accordingto claim 23, wherein ion exchange is carried out as the step ofpurifying.
 26. The method for producing a polishing agent according toclaim 14, wherein an alkaline silica sol having a pH in the range of 8to 13 is used as the prepared silica sol.
 27. The method for producing apolishing agent according to claim 25, wherein an alkaline silica solhaving a pH in the range of 8 to 13 is used as the prepared silica sol.28. The method for producing a polishing agent according to claim 14,wherein a silica sol containing spherical-silica particles having anaverage particle size in the range of 11 to 13 nm is used as theprepared silica sol.
 29. The method for producing a polishing agentaccording to claim 27, wherein a silica sol containing spherical-silicaparticles having an average particle size in the range of 11 to 13 nm isused as the prepared silica sol.
 30. The method for producing apolishing agent according to claim 14, wherein after the ion exchange iscarried out, the silica sol has a pH in the range of 2 to
 4. 31. Themethod for producing a polishing agent according to claim 29, whereinafter the ion exchange is carried out, the silica sol has a pH in therange of 2 to
 4. 32. The method for producing a polishing agentaccording to claim 14, wherein the silica sol after the acid is addedhas a pH in the range of 10 to
 12. 33. The method for producing apolishing agent according to claim 31, wherein the silica sol after theacid is added has a pH in the range of 10 to
 12. 34. The polishing agentproduced by the method according to claim
 14. 35. The polishing agentproduced by the method according to claim
 33. 36. A method for producinga silicon wafer comprising at least a polishing step, wherein as thepolishing step, the wafer is polished by contacting and rubbing with apolishing pad with providing the polishing agent produced by the methodaccording to claim 14 to the polishing pad.
 37. A method for producing asilicon wafer comprising at least a polishing step, wherein as thepolishing step, the wafer is polished by contacting and rubbing with apolishing pad with providing the polishing agent produced by the methodaccording to claim 33 to the polishing pad.